New evolved wireless communication systems are constantly being developed. By way of example, so-called fifth generation, 5G, systems are currently being discussed, developed and/or deployed. 5G, also referred to as New Radio, NR, is currently viewed primarily as a change to the radio specifications, although other aspects may be concerned.
5G, as an illustrative example, is expected to operate in a wide range of frequency bands, probably using also very high frequency bands compared to 4G. This implies, for example, lower diffraction and higher outdoor-to-indoor penetration losses, which means that signals will have more difficulties to propagate around corners and penetrate walls. Also, the initial deployment of 5G will be rather spotty.
The state-of-the-art integration between two different Radio Access Technologies, RATs, is normally based on so-called hard handover. The major drawbacks with inter-RAT hard handover, e.g. between 3G and 4G, are the rather long delay and service interruption as well as the low reliability. A tighter integration with evolved LTE may therefore be crucial in order to ensure ultra-high reliability and extreme bit rates in a 5G system. This may also hold true for other RATs.
A plausible alternative is to use a solution based on Multi Connectivity, MC, or Dual Connectivity, DC. In general, MC, of which DC is a special case, implies the possibility of multiple (dual) connections that are maintained in parallel. Usually, but not limited thereto, the parallel connections are based on different RATs. It is also possible to provide Multi Connectivity based on one and the same RAT.
FIG. 1 is a schematic diagram illustrating an example of the general principles of Multi/Dual Connectivity in a wireless communication system. In this example, the wireless communication system comprises a first network unit and a second network unit that are both connected to a third network unit using a first connection, between the first network unit and the third network unit, and using a second connection, between the second network unit and the third network unit. Additional connections to the third network unit from further network units (not explicitly shown in FIG. 1) may be provided.
FIG. 2 is a schematic diagram illustrating an example of a Multi/Dual Connectivity system based on two or more radio units being simultaneously connected to a wireless device. The radio units may be connected to or at least partly implemented in a central unit.
For example, in 5G DC, both User Plane, UP, and Control Plane, CP, are normally connected to both LTE and 5G and the UP data is aggregated (or split) at the Packet Data Convergence Protocol, PDCP, layer. Typically, this means that the so-called “bearer split” option (also called 3C) is employed, i.e. the bearer is split in the master eNB at the PDCP layer. Dual connectivity increases the user throughput (due to UP aggregation), especially at low load and increases the reliability (due to CP diversity).
However, MC or DC does not increase the coverage of the user plane data like solutions such as Coordinated Multi Point, CoMP, soft handover and multi-flow. All these solutions transmit the same UP data over all links and thereby increases the coverage. CoMP and Soft handover rely on a synchronized transmission (and reception) and maximum ratio combining (MRC) of the signals (i.e. combing of the symbols). However, for LTE and 5G (NR) this can be very difficult due to different transport formats, pilots, waveforms, numerology etc. Also, both CoMP and soft handover requires very good backhaul (X2) and quite synchronized networks. Therefore, solutions like HSPA multi-flow may be a solution for LTE-5G tight integration, as well as NR-NR multi-connectivity, enabling coverage extension due to selection ratio combing (SRC). This is not as good as CoMP/Soft handover which enables MRC but can still give large benefits especially in situations with very fast and varying channels.
A main problem with Multi Connectivity in general, and LTE-5G dual connectivity in particular, is that it does not by itself increase the coverage of the user plane data. One option is to multiplicate/duplicate the packets, i.e. transmit the same packets over two or more connections, or more generally transmit packets comprising the same information over two or more connections.
By way of example, for LTE-5G, the same PDCP packets may be transmitted in both connections/links/RATs. In HSDPA multi-flow, the Radio Link Control, RLC, packet flow may be split over two links and it is also possible to duplicate the RLC packets over the two links.
A major problem with sending the same PDCP packets from more than one node is that one of the nodes may need much longer time to transmit the PDCP packet than the other nodes(s) due to bad coverage or different link bit-rates. This has the disadvantage that the UE therefore need to wait for the worst node packet transmission before requesting a retransmission. This both creates the need for increased buffer size in the UE, extra packet delays and unnecessary interference. The PDCP receiver entity discards the redundant PDCP PDUs by checking the sequence number at PDCP header. However, this only solves the problem to some extent. This solution still induces extra delay and extra interference.
Another problem with multiplicated/duplicated transmissions is that it may waste resources if one link is slower than the other. If for example NR and LTE are used, then the NR link (when in good coverage) may have a bitrate far exceeding that of LTE. This means that LTE may fall behind and transmit RLC PDUs which were already transmitted by NR several TTIs ago. In this case, the duplicated transmissions will eventually be so late, that they are more or less useless. This problem is illustrated in FIG. 3 where different RLC packets are transmitted in the first TTI and already in the second TTI the slow link is transmitting RLC PDUs which were transmitted by the fast link in the first TTI. If this would be continued for a number of TTIs, it is easy to see that the slower link is useless and that PDCP duplication in similar situations will be very resource consuming with very little benefits.